1. Field of the Invention
The present invention relates to a micro-pattern measuring apparatus such as an SEM (scanning electron microscope) for measuring the size of a micro pattern formed on a semiconductor device such as an LSI, a VLSI, or a ULSI. In particular, the present invention relates to an in-line process monitor for measuring the size of a micro pattern during manufacturing processes of semiconductor devices.
2. Description of the Prior Art
FIG. 1 shows a conventional SEM employing an electron beam to measure the size of a micro pattern formed on a semiconductor device such as a VLSI. A semiconductor substrate (wafer) 101 has a micro pattern 102. The SEM emits a thin electron beam 103 to the pattern 102, receives a secondary electron signal from the pattern 102, and generates a luminance modulation signal for a CRT (cathode ray tube) according to the intensity of the secondary electron signal, to display an image of the pattern 102 on the CRT in synchronization with the scanning of the pattern 102 with the electron beam 103. Then, the size of the pattern 102 is measured on the displayed image of the pattern 102.
If the micro pattern 102 is made of material that causes no charge-up phenomenon, there will be no problem. When the pattern 102 is made of photoresist or insulation material such as silicon, silicon oxide (SiO.sub.2), or silicon nitride (Si.sub.3 N.sub.4), the charge-up phenomenon occurs. FIG. 2 shows the secondary electron signal from the micro pattern 102. This signal has a slope 104A corresponding to a left edge 102A of the pattern 102, and a slope 104B corresponding to a right edge 102B of the pattern 102. Since the slopes 104A and 104B are asymmetrical, it is impossible to correctly detect the right edge 102B. This problem also occurs even if wiring material is silicide or aluminum, if the material is covered with photoresist or insulation material. The micro pattern 102 covered with photoresist or insulation material such as silicon oxide causes the charge-up phenomenon when the electron beam 103 irradiates the right edge 102B of the pattern 102 and a plain area 104. Consequently, the signal's slope 104B corresponding to the right edge 102B of the pattern 102 becomes dull as encircled in FIG. 2, and the slopes 104A and 104B become asymmetrical. This results in erroneously detecting the right edge 102B, to incorrectly measure the size of the pattern 102. To avoid the charge-up phenomenon, prior-art techniques generate an electron beam with a reduced acceleration voltage or with a reduced current quantity, or deposit metal thin film over the pattern 102.
Reducing the acceleration voltage, however, lowers the intensity of the electron beam or increases the diameter thereof, to thereby deteriorate resolution. Similarly, reducing the electron beam current deteriorates the S/N ratio of a secondary electron signal. Namely, the acceleration voltage or the electron beam current must not be decreased too low. Depositing metal over the micro pattern 102 will hide, damage, or deform microstructures on the substrate 101. The charge-up phenomenon frequently causes measurement trouble because recent technology provides very fine patterns and complicated structures. When a measurement object is a hole surrounded by photoresist, the charge-up phenomenon is easily caused by charged electrons that have no place to escape, to make the hole unmeasurable.
Depositing a metal thin film such as a silver thin film over a measurement object causes a contamination problem later. Accordingly, this technique is improper for the in-line measurement of micro patterns.